These two definitions are both right. In the popular T.I. song, What You Kno, T.I. uses a metaphor to bring together the act of sellin cocaine three kilograms (keys) at a time with the act of calling to arrange the deal. When he says "key by the three" he is stating that he has three keys of cocaine and also that he is using his nextel to arrange the deal.

"I gotta key by the three, when i chirp shawty chirp back"
T.I.--What You Know

Three kilograms of cocaine. A key is standard slang for a kilogram of drugs, most often cocaine. By stating "by the three," one is saying that they buy cocaine 3 kilograms at a time with the intention of dealing. The following lines show this more. The statement of a Louis Nap Sack is in reference to a Louis Vuitton bag where he holds all of the cocaine or "work" to be dealt.

The other definition for key by the three is wrong, although the "when I chirp shorty chirp back" is in reference to cell phones.

Key by the three, when I chirp shorty chirp back.
Louis Nap Sack
Where I'm holding all the work at.